The present invention relates to a method and a system for purifying and concentrating spent and diluted sulphuric acid from nitration processes, in which nitric acid is used as a nitrating medium in the presence of sulphuric acid. Such a diluted sulphuric acid is referred to below as a rule as “waste acid from the nitration process”.
Nitration processes play a key role in the chemical industry for the production of nitro compounds and are carried out on an industrial scale. In most cases, the nitro compounds produced serve as valuable intermediates owing to the numerous potential reactions of a nitro group and the secondary products which can be produced therefrom.
Particularly important is the nitration of aromatic compounds, for example of toluene, with production of nitro aromatics, in particular of dinitrotoluene (2,4-dinitrotoluene; DNT), since aromatic nitro groups can be converted by reduction in a simple manner into amino groups, which in turn can be subjected to numerous further reactions. A particularly important further processing, for example of DNT, is that to give toluene diisocyanate (TDI) after an initial reduction of DNT to toluenediamines and a subsequent reaction thereof with phosgene, or by direct reaction of the DNT with CO. TDI is one of the most important aromatic diisocyanates for the production of polyurethanes.
The introduction of one or more nitro groups into an aromatic compound, such as, for example, toluene, is effected as electrophilic substitution with the aid of nitric acid mixed with sulphuric acid, for example in one stage with nitrating acid (this term refers to mixtures of concentrated sulphuric and nitric acid of different compositions) or, in the case of a dinitration, as in the production of DNT, optionally also in two stages with acids of different strengths, in a typical two-stage process toluene being nitrated to give mononitrotoluene (MNT) in the first stage, with the use of a less concentrated optionally worked-up waste acid from the second stage, and a further nitro group being introduced into this MNT in the second stage with the use of a highly concentrated nitric acid with formation of DNT (cf. for example EP 155 586 A1).
While the nitric acid fraction of the nitrating acid is consumed to a very high proportion during the introduction of the nitro groups as substituents on the aromatic ring, the simultaneously present concentrated sulphuric acid is only diluted and contaminated by the resulting water of reaction. It forms the “waste acid from the nitration process” mentioned at the outset, which typically comprises mainly a more than 70% by mass sulphuric acid and water, and smaller proportions of nitric acid, nitro-organic compounds, for example mononitrotoluene (MNT) and dinitrotoluene (DNT), and nitrous constituents in the form of, for example, nitrosylsulphuric (abbreviated below to NSS).
It is then an urgent requirement of modern economic process management to work up this waste acid from the nitration process and to convert it into a concentrated sulphuric acid which can be reused in the process. Use of fresh sulphuric acid for the nitration process has long been unacceptable for economic and environmentally relevant reasons.
Reconcentration of the stream of waste acid from the nitration process to sulphuric acid concentrations between 89% by mass and 94% by mass is therefore regularly carried out today, and it must be an aim to achieve such reconcentration with as little energy input as possible and the formation of as small amounts as possible of waste streams which are contaminated with chemicals and have to be discharged from the process.
A high degree of concentration of sulphuric acid to max. 96% by mass sulphuric acid content, which is expediently effected under reduced pressure conditions in the final stages because of thermodynamic requirements in the process, has for decades been part of the prior art with the development of highly corrosion-resistant materials suitable for this purpose, such as tantalum, enamel and glass, for the evaporation at high temperatures. However, the fact that the preferred evaporator material tantalum can be used only at a maximum boiling point of less than 208° C. of the waste acid from the nitration process which is to be concentrated is to be regarded as a limitation.
An early method for concentrating spent dilute sulphuric acid to give relatively pure acid of about 96% by mass was the so-called Pauling method [Bodenbrenner, Von Plessen, Vollmüler, Dechema-Monogr. 86 (1980), 197]. However, this method was energy-intensive and had, as a further disadvantage, a high level of formation of SO2 and NOx compounds, owing to a strong oxidative decomposition of the organic compounds present in the waste acid from the nitration process at the required high temperatures.
The particular problem in the concentration of waste acids from nitration processes lies in the foreign constituents, originating from the nitration, in the acid, in particular in the form of compounds containing nitrogen-oxygen groups, such as nitric acid, various organic compounds, mainly MNT and DNT, and the dissolved nitrous constituents in the form of nitrosylsulphuric acid (NSS), the content of which is as a rule stated as the content of nitrous acid (HNO2).
Since, by virtue of their character, said substances are either potential feedstocks or incompletely isolated products or intermediates, it is of course desirable to be able to recycle them very substantially, as in the case of the sulphuric acid, to the nitration process and to keep the losses of these feedstocks, intermediates and end products as small as possible. The methods of the prior art were however still suboptimal in this respect, in particular if it is considered that such potentially useful products can also be found in other product streams of a process for the nitration of aromatic compounds, for example in the wash water of the end product of the toluene nitration process which is isolated in crystalline form, the DNT, which streams have to date been discharged as waste streams from the nitration process and worked up separately. A DNT wash water of said type is acid-containing, and is therefore also referred to as wash acid, and may contain, for example, a typical composition of 10-20% by mass of HNO3, 5-12% by mass of H2SO4 (18-35% by mass of total acid) and dissolved nitro-organic compounds (DNT, MNT). The term “mixed acid” is also used for this wash acid.
The presence of said nitrogen-oxygen compounds, in particular of the nitro-organic compounds, in the waste acid from the nitration process means that the recovery and concentration of the sulphuric acid fraction present therein is associated with particular technical difficulties. Thus, the proportions of the nitrated aromatics, which are sparingly volatile solids which have a low solubility in aqueous media and may be precipitated from these, may be deposited on parts of the plant, block them and thus interfere with the overall method. The proportions of nitric acid and nitroso compounds on the other hand are relatively volatile and can pass over into evaporation streams, from which however they are difficult to recover and may constitute an environmental risk.
Regarding the individual troublesome ingredients of a waste acid from the nitration process, the problems which they give rise to in the concentration of the waste acid from the nitration process with recovery of the concentrated sulphuric acid are described below, known methods of the prior art for solving the specific problems associated with these ingredients being discussed at the same time:
a. Nitric Acid
In the individual evaporation stages of the working-up of waste acid from the nitration process for sulphuric acid concentration, the nitric acid present in the feed is finally virtually completely evaporated from the concentrated sulphuric acid, owing to its comparatively high volatility, therefore mainly enters the vapour condensate and, if no special measures are taken, inevitably enters the waste water to be discharged. In the case of some waste acids from the nitration process, the content of HNO3 in the feed may easily be 1-2% by mass according to the nitration process, in particular when the nitration is carried out with highly concentrated nitric acid of about 98% by mass—99% by mass instead of with azeotropic nitric acid.
Such high proportions of HNO3 in the waste acid from the nitration process would lead not only to the loss of this nitric acid but also excessively high pollution of the waste water with nitrate. The latter is no longer acceptable today for environmental protection reasons.
In the past, as a rule the waste acid from the nitration process was therefore fed in a pre-concentration stage to the upper part of a stripping column operated with steam by the countercurrent method, in order virtually completely to eliminate the nitric acid from the sulphuric acid. A further desired effect of this pre-concentration by means of a stripping column consisted in being able simultaneously also to remove a part of the other interfering products in the waste acid from the nitration process by stripping, such as, for example, a part of the nitro-organic compound load. The energy for such a pre-concentration stage, which is predominantly operated under atmospheric pressure, was supplied either indirectly by a separate heater or by direct steam introduced into the stripping column, depending on the method.
A very aqueous DNT/MNT- and HNO3-containing heterogeneous solution was inevitably obtained as a top product of the pre-concentration stage, after the total condensation thereof, from which solution the insoluble organic constituents precipitated as solids had first to be separated off by gravitational force (allowing to settle, centrifuging). In order to recover the nitric acid present in the liquid phase obtained, an additional subazeotropic rectification stage, which generally operated in the same way as the pre-concentration stage at atmospheric pressure, was integrated into the working-up method in order to obtain an about 45-50% nitric acid in the bottom product of such rectification. This can be used in many nitration processes without further higher concentration, as can the organic compounds separated off (mainly DNT), by recycling directly into the nitration.
A relatively acid-free waste water which was suitable for being used at least partly as wash water for the DNT washing could be obtained as a top product of the nitric acid rectification.
That step of a special nitric acid recovery method which is integrated into the working-up of the waste acid from the nitration process is known by the technical term “nitric acid pre-concentration”, abbreviated by the letters “NAPC”.
A substantial disadvantage of this NAPC stage is that, as a result of the rectification, the water to be discharged from the stripping column evaporates a second time and has to be condensed. Moreover, there is the danger that at vapour condensate temperatures below 55° C., the unprecipitated DNT will settle as a solid in parts of the system and will thus lead to considerable difficulties.
The NAPC stage is used in known methods even when the abovementioned acid-containing wash water from the DNT washing, which is a sulphuric acid/nitric acid mixture comprising 18-35% by mass, generally 20-30% by mass, of total acid has to be worked up. Since, in such an acid-containing wash water to be worked up, nitrous constituents and DNT are likewise present in the feed, this wash acid is comparable with the vapour condensate of the pre-concentration stage and has similar problems.
Nitrosylsulphuric acid NHOSO4 (HNO2)
The nitrosylsuiphuric acid (NSS), which scarcely exists in dilute aqueous sulphuric acids but is very stable in highly concentrated sulphuric acids of more than 82% by mass, is formed in the nitration process and may be present in amounts up to more than 5% by mass in the waste acid from the nitration process, which acid is to be concentrated. It embodies the potential of the nitrous constituents present in the waste acid from the nitration process, the content of which constituents is stated as nitrous acid HNO2.
The reason is that the analytical determination of the NSS in the waste acid from the nitration process is usually effected in very dilute solution, according to the following equation, as HNO2:HNOSO4+H2O→H2SO4+HNO2  a.
In the case of the thermal concentration of sufficiently water-containing waste acid from the nitration process, the NSS present therein is to a great extent decomposed according to the following equation:2HNOSO4+H2O→2H2SO4+NO2+NO  a.
If the resulting nitrogen-dioxide reaches the condenser region, for example of the pre-concentration stage, by stripping, a considerable amount of fresh very dilute nitric acid is very rapidly formed therefrom and has to be worked up together with a large proportion of water in the NAPC step.
The NO gas present in the waste acid from the nitration process and newly formed according to the abovementioned equation is insoluble and, owing to the lack of oxygen in the system, passes unchanged through the condensers. As is in any case usual in the industrial processes today, this NO-containing gas is fed to a further working-up stage for purification and recovery, for example to an NO absorption stage for the production of further subazeotropic nitric acid.
In spite of the difficulties which the oxides of nitrogen in the top product of the stripping column present for the pre-concentration of the sulphuric acid, the decomposition of the NSS(HNO2) in this concentration stage is desired since the NSS no longer decomposes above the abovementioned limit of 82% by mass of sulphuric acid in the further concentration in which the sulphuric acid contents increase further. If the NSS were not decomposed in the pre-concentration stage, it would be circulated with the concentrated sulphuric acid, and its content would increase unnecessarily during the method with progressive nitration and concentration of the waste acid from the nitration process.
A pre-concentration stage according to the prior art to date, as was described herein, does not however optimally perform the function of the decomposition of the NSS.
Nitro-Organic Compounds, Mainly DNT
Most problems in the concentration of the aqueous feed acid (i.e. the waste acid from the nitration process) are presented by the nitro-organic compounds dissolved in said acid, chiefly the DNT. Owing to the limited miscibility of the nitro-organic compounds in the aqueous feed acid, MNT and DNT are steam-volatile and, in spite of the high boiling points of the pure substances, can in principle be stripped out from the acid by steam. However, in the case of DNT, the highest-boiling compound of the nitro-organic compounds, stripping with stripping steam in the customary pre-concentration stage of the prior art takes place only to an insufficient extent.
The fact that considerable amounts of DNT were still present in the waste acid from the nitration process even after its pre-concentration had the result in the known methods that, in the following stages for the concentration of the waste acid from the nitration process with reduced-pressure operation at about 100 mbar abs or below, further DNT entered the vapour condensate of these reduced-pressure stages, owing to the thermodynamically improved conditions for the volatilization of the DNT. Since the vapours of the reduced-pressure stages have to be condensed at relatively low temperatures of <45° C. owing to the reduced pressure in the system, the undissolved DNT is precipitated as a solid from the vapour condensates and presents considerable problems for the continuous sulphuric acid concentration process.
In order to avoid the problems which are caused at reduced pressure by the DNT precipitation in the condensates of the vapours of the concentration of waste acid from the nitration process, EP-A 0 155 586 A1 proposed, in a two-stage process for DNT production, in which the dilute waste acid from the initial process stage of mononitration is fed directly to a reduced-pressure concentration stage, additionally spraying a defined amount of MNT into the condensers at selected points in order to reduce the setting point of the DNT fraction and to prevent the DNT crystallisation. This spraying with MNT is complicated and introduces MNT into the process streams at those points of the concentration process where it can have a thoroughly troublesome effect. It is to be regarded as particularly disadvantageous that the total waste water which is produced during the concentration of the waste acid from the nitration process and from the additional foreign steam which may be used for process engineering reasons is additionally contaminated with organic compounds in such a procedure.
To take this deficiency into account, DE 196 36 191 A1 proposed, in the working-up of the waste acid from the nitration process, introducing a stage for the purification of the waste acid by stripping with as much steam as possible upstream of the reduced-pressure concentration, in order to ensure that the nitro-organic compounds, in particular the poorly strippable DNT having a high setting point, are virtually completely eliminated from the outflowing waste sulphuric acid in this stage (residual content <20 ppm), so that, in the subsequent concentration stages at 100 mbar (a) or below, problems due to the DNT precipitates described can no longer occur in the aqueous condensates of these stages.
A major deficiency of this solution is that it is not possible to produce the large amounts of steam required for the DNT stripping by self-evaporation from the waste acid from the nitration process which is used, even if the stripper and its evaporator are operated for this purpose under optimum reduced-pressure conditions under which the dew point or boiling point of the water is still above the temperature at which DNT begins to precipitate. Thus, the stripping of DNT with the desired effectiveness requires the use of a considerable amount of additional stripping steam, which causes the steam consumption of the overall method to increase greatly and moreover leaves a marked increase in the amount of waste water.
In the above discussion of the product streams of a system for the nitration of aromatics, for example for the nitration of toluene for the production of DNT, and for the working-up of the resulting waste acid from the nitration process, it was not taken into account that a highly concentrated nitric acid (75 to 99.9% strength, usually 98-99% strength) is also required in said systems for achieving the desired dinitration, in particular either for the production of a concentrated “nitrating acid” in the above sense or for the introduction of a second nitro group into the mononitrotoluene (MNT) produced in a first stage with the use of an approximately azeotropic nitric acid, in a two-stage procedure. This highly concentrated nitric acid is produced in a separate system which is coordinated with the actual system for carrying out the nitration and serves only for carrying out the process designated as NACSAC process (“nitric acid concentration sulphuric acid concentration”) and intended for achieving a high degree of concentration of a dilute, usually azeotropic nitric acid and reconcentrating the extracting medium sulphuric acid.
In this system assigned to the actual nitration process, an optionally contaminated approximately azeotropic nitric acid obtainable as a feed stock and having a concentration in the range of 40 to 70% by mass, in particular of about 67% by mass, is concentrated by countercurrent extractive rectification. In the extractive rectification, as a rule a concentrated sulphuric acid having a concentration in the range of 86% by mass to 90% by mass is used for binding the proportions of water of the approximately azeotropic nitric acid, which concentrated sulphuric acid is diluted to about 70% by mass during the extractive rectification by uptake of the water from the nitric acid to be concentrated. In order to be able to recycle this dilute sulphuric acid to the extractive rectification, it must be reconcentrated. This reconcentration is usually effected in a particular part of the total NACSAC system for nitric acid concentration. The circulated sulphuric acid used in this system as an extracting medium is also referred to as “circulating acid” or “recycle acid” and is not part of the mass balance of the actual nitration process. Additional embodiments for achieving a high degree of concentration of nitric acid by extractive rectification using the extracting medium sulphuric acid and for reconcentrating the extracting medium are to be found in the patents EP 1 284 928 B1 and U.S. Pat. No. 6,969,446 B1 of the applicant.